Proteoglycans are proteins conjugated to one or more glycosaminoglycan (GAG) chains. These proteins are distributed inside cells, on the cell membrane and in the extracellular matrix serving a variety of functions: cartilage matrix formation; the structural organization of tissues; organizations of basement membranes; regulating the role of secretory vesicles; binding of cytokines, chemokines, growth factors, and morphogens; protease receptors and protease inhibitors; co-receptors, tyrosine-kinase-type growth factor receptors; as endocytic receptors; facilitate cell attachment, cell-cell interactions, and cell motility as well as cell migration.
The malaria parasite Plasmodium falciparum utilizes host cell proteoglycans in almost all stages of its complex life cycle. The sporozoite infects hepatocytes in the liver through surface-expressed circumsporozoite protein interacting with highly sulfated heparan sulfate proteoglycans (HSPG). Merozoite infection of the erythrocytes is mediated by EBA-175 binding to sialic acid on glycophorin A. In addition, a number of Plasmodium falciparum Erythrocyte Membrane Protein 1 (PfEMP1) proteins, mediating host endothelial adhesion, have been described as glycan-binding. One of these is VAR2CSA, which is a unique member of the PfEMP1 protein family. VAR2CSA binds with high affinity to an unusual, low-sulfated form of chondroitin sulfate A (CSA), attached to proteoglycans, so called Chondroitin Sulfate Proteoglycan (CSPG), in the intervillous spaces of the placenta. VAR2CSA is a large multidomain protein (350 kDa) expressed on the surface of P. falciparum-infected erythrocytes (IEs), and the VAR2CSA-CSA interaction is responsible for placenta specific sequestration in placental malaria (PM). Importantly, recombinant full-length VAR2CSA ecto-domain from FCR3 and 3D7 type parasites has shown affinity for CSA in the low nano-molar range.
CSA belongs to the family of glycosaminoglycans (GAGs), which are linear polymers of alternating amino sugars and hexuronic acid residues, attached to proteoglycans. There are several types of GAGs including; chondroitin sulfate (CS), dermatan sulfate (DS or CSB), heparan sulfate (HS) and heparin. While the polysaccharide backbone of these GAGs is simple, considerable diversity arises in modifications such as sulfation and uronate epimerization. This is the basis for the wide variety in domain structure and biological activities of different GAGs.
CS interacts with many important factors such as growth hormones, cytokines, chemokines, and adhesion molecules and is thought to be involved in structural stabilization, cytokinesis, cell proliferation, differentiation, cell migration, tissue morphogenesis, organogenesis, infection, and wound repair. CS chains are composed of alternating units of N-acetyl-D-galactosamine (GalNAc) and glucuronic acid residues. Glucuronic acid can be sulfated at its C2 position and GalNAc can be sulfated at C4 and/or C6, giving rise to various disaccharide units. Varying modifications of the sugar backbone allows structural and functional heterogeneity of the CS chains. Placenta adhering P. falciparum parasites specifically associate with low sulfated CSA with sulfation only at C4 of GalNAc.
Early studies pinpointed CSA as being responsible for IE sequestration in the placenta. The specific receptor was however not known. Upon further research it was found that the human placenta contained three distinct types of chondroitin sulfate proteoglycans (CSPG), but that the IE adhered specifically to low sulfated CSPG in the intervillous spaces. What is special for this type of CSPG is that only 2-8% of the disaccharide units are C4 sulfated. In an accompanying study, aimed to identify the specific structural requirements for the CSA, it was found that parasite adhesion to CSPG is inhibited by CSA containing between 30-50% C4 sulfation, with the remaining 50-70% disaccharide units being unsulfated. The minimal inhibition of binding requirements for CSA was shown to be a dodecasaccharide (six disaccharides) containing a minimum of 2-3 or 4-5 C4 sulfated disaccharide units.
Chondroitin sulfate proteoglycan 4 (CSPG4), also known as High Molecular Weight-Melanoma Associated Antigen (HMW-MAA) or melanoma-associated chondroitin sulfate proteoglycan (MSCP), is a cell surface proteoglycan which has been shown to be expressed by melanoma cells.
CSPG4/MSCP/HMV-MAA is a large proteoglycan characterized by having CS chains on the protein backbone. The sulfation of these CS chains seems to be primarily on C4 of GalNAc (CSA), although the degree of sulfation is not known.